The present invention relates to an arrangement for providing mobile stations in (or outside) a cellular communication system with radio coverage when they are in a mobile vehicle. The invention also relates to a base station providing mobile stations in a mobile vehicle with radio coverage, to a cellular communication system in which radio coverage is provided for mobile stations in a mobile vehicle and to a method of providing radio communication for mobile stations in a mobile vehicle.
The user of a mobile station who is inside a mobile vehicle such as for example a train or similar will notice that the radio coverage sometimes is poor or even non-existent. This is a problem since when travelling it may be impossible to make or to receive any calls at certain parts of the track, ongoing calls may be dropped and the quality may also be varying. If, for example, a part of the track lacks radio coverage, such as for example in a tunnel, then the inside of the vehicle, e.g. the train, will experience the same lack of coverage and all ongoing calls will be dropped. It will of course also be impossible to make or receive any calls.
There are several problems associated with the provision of radio coverage to mobile stations in a mobile vehicle. Often such a mobile vehicle comprises a metal structure, inside which the mobile stations are located. The radio frequent signal, uplink (towards the base station) as well as downlink (from the base station to the mobile stations) will therefore be attenuated which has as a consequence that a mobile station will have to transmit on a higher power level than would have been necessary for example in free air. This means that the available talk/standby time will be decreased which is clearly disadvantageous. Furthermore the call will have a lower signal strength. This means that a call has a higher probability of being dropped close to the cell border. Another problem associated with mobile stations in a mobile vehicle is that all mobile stations in a specific network will be handed over to the next cell at about the same location which produces a high load on the network as such. The individual calls also run a certain risk of being dropped during a handover. Another problem is that all the cells that the mobile vehicle passes have to be dimensioned for the mobile vehicle, or rather the mobile stations that may be travelling with it, even though the cell otherwise might be sparsely populated or hardly populated at all which as such leads to a poor and unsatisfactory usage of equipment.
Numerous attempts have been done to find a satisfactory solution to these problems. Various solutions are for example based on the use of various kind of repeaters in order to enhance the radio coverage inside the train from the surrounding cell it passes through. It is common to all such known solutions that they make use of the existing cellular network and they can not make the radio environment inside the mobile vehicle any better than it is outside the mobile vehicle, and, if a part of the track lacks coverage, the situation will be the same inside the mobile vehicle.
U.S. Pat. No. 5,519,761 relates to an airborne radio telecommunication system. According to this document an aircraft can be provided with a repeater, as discussed above, but in a particular embodiment the repeater can be replaced by an airborne base station. The airborne base station has capability of registering telephones at the aircraft and the airborne base station then registers the telephone in the ground based system. Thus, once a telephone has been registered in the airborne base station, the airborne base station searches for the strongest pilot signal from a ground base station and registers with it therefore, also here the existing communications system is used, an airborne base station substantially only being used for temporary registration purposes.
What is needed is therefore an arrangement that efficiently can provide a mobile station with radio coverage when the mobile station is inside a mobile vehicle, particularly a mobile vehicle comprising a metal structure, in a satisfactory manner, particularly without requiring the mobile station transmitting using a particularly high power level and with a satisfactory or high strength and carrier to interference ratio. An arrangement is also needed through which the dropping of calls due to the mobile vehicle having a poor radio coverage can be reduced or avoided. Particularly a base station is needed trough which the above mentioned objects can be achieved. Still further a cellular communication system is needed through which the above mentioned objects are fulfilled and through which the load on the network can be appropriately dimensioned without taking into consideration temporary situations such as for example a mobile vehicle passing which considerably changes the circumstances in the network as far as load etc. is concerned. Furthermore a communication system is needed through which the equipment in the system can be efficiently used and through which the load is not unduly high, i.e. for example because of a number of mobile stations having to perform a handover at substantially the same time at substantially the same place. Particularly an arrangement and a system respectively is needed through which the risk of calls being dropped due to poor radio coverage or due to handover is reduced.
A method of providing a mobile station with the radio coverage when the mobile station is within the mobile vehicle is also needed through which the above mentioned objects are accomplished.
Therefore an arrangement for providing mobile stations, of a cellular communication system, in a mobile vehicle with radio coverage, said mobile vehicle being mobile within or outside the cellular radio network which comprises a number of fixed radio base stations and a number of switching arrangements. The arrangement comprises a mobile base station arranged within the mobile vehicle which serves at least one cell provided in said mobile vehicle, internal antenna means arranged within the mobile vehicle for providing communication with the mobile stations in the mobile vehicle, external communication means for providing communication between the mobile vehicle and the core network (the fixed part of the network) and, when the mobile base station is (or intends to become) stationary within a fixed cell, at least for a given time period, it is docked with said fixed cell such that handovers are allowed between the mobile base station cell and the said stationary cell. Particularly handovers are not allowed when the mobile cells served by the mobile base station(s) are not within coverage of and stationary etc. within a fixed cell. Thus the inventive concept comprises the provision of one or more mobile cells within the mobile vehicle. Through using an appropriate internal antenna arrangement, the entire mobile vehicle will have an excellent radio environment. Such an antenna arrangement is advantageously so arranged that the distance between any mobile station within the mobile vehicle and the antenna arrangement is as short as possible leading to the requirements as to required power level are low which both results in low power consumption in the respective mobile stations and it also has as a result that only a little radio frequency energy is radiated into surrounding cells. The mobile cell(s) within the mobile vehicle will therefore not be noticed by the cells through which it travels. Particularly handovers between a mobile cell served by the mobile base station and a fixed cell are allowed when the mobile vehicle is detected or indicates that it will becomes stationary in a fixed cell in addition to when it actually is stationary within a fixed cell. Even more particularly, a neighbouring relationship is established between a mobile cell and a fixed cell when the mobile cell intends to stop in a fixed cell, according to which neighbouring relationship handovers are allowed between the mobile and the fixed cells respectively. When referring to fixed cells, cells are meant which in some way are predefined as, or characterised through, allowing docking of a mobile cell, i.e. allowing establishment of a neighbouring relationship with a mobile cell. Of course a mobile vehicle generally also moves through other cells. These, however, are supposed not to allow any docking of a mobile cell and are not interesting for the present inventive concept.
In a particular implementation of the above embodiment, a mobile cell broadcast information message is broadcast when a mobile base station or a mobile cell is stationary within a fixed cell controlled by a fixed base station indicating said fixed cell as the only neighbour to the mobile cell. Even more particularly a fixed cell broadcast information message is broadcast, when a mobile base station is stationary within a fixed cell as referred to above, indicating at least said mobile cell as neighbour to the fixed cell (generally among other conventional fixed cells).
In a particular implementation broadcast message information is sent out also when a mobile base station mobile cell(s) indicates its intention to stop in a fixed cell indicating said fixed cell and said mobile cell respectively as neighbours and allowing handovers between said cells. Particularly, when a mobile base station is outside coverage of any of the fixed cells and does not intend to become stationary within any of said fixed cells, mobile cell broadcast information and information, e.g. in the form of Condition Report Information (CRI) or similar is sent out indicating that a mobile cell has no neighbours and fixed cell broadcast information does not contain any information about the mobile cell. Information is provided to all mobile stations within the mobile cell indicating no neighbours, updated information being provided to mobile stations with ongoing calls indicating the current situation.
According to a particular embodiment handovers are only possible for the mobile base station as actually stationary within a fixed cell. In a particular embodiment a set of virtual mobile cells are indicated as permanent neighbours to a fixed cell and most particularly, when a mobile base station is stationary within a fixed cell controlled by a fixed base station, mobile cell broadcast information message is broadcast indicating said fixed cell as the only neighbour to the mobile cell, handovers being controlled by a switching arrangement, e.g. a mobile switching center handling mobile cells, preventing handovers between the fixed cell and the mobile cell unless the mobile cell actually is stationary within said fixed cell or somehow has indicated its intention to become stationary within the fixed cell.
In a particular implementation the virtual mobile cells are shown as permanent neighbours in the fixed cell broadcast information message whereas the mobile cell broadcast information message is adapted to the current situation indicating a fixed cell as a neighbour only when the mobile cell is, or intends to become stationary within the fixed cell, mobile stations within the mobile cell with ongoing calls being provided with updated information.
In a particular embodiment, implementable to any of the preceding embodiments, frequency hopping is implemented in the mobile cell to reduce interference produced by external conventional, fixed cells. As referred to earlier, the mobile cell will be not noticed by conventional fixed cells it passes through.
In an alternative implementation, in a mobile cell controlled by a mobile base station intra-cell handover to a frequency of the lower interference level is implemented to reduce interference produced by conventional, fixed cells.
In an advantageous embodiment the mobile vehicle includes means for establishing its location (in the radio network) which may take a number of different forms.
In a particularly advantageous embodiment the mobile base station comprises frequency adaption means for adapting the frequencies used in the mobile cell in relation to the frequencies used in a fixed docking cell so that the mobile cell uses a different set of frequencies.
The inventive concept is applicable to substantially all kinds of mobile vehicles such as trains, aircrafts, ferry-boats, ships or undergrounds etc.
Therefore also a base station is given which provides mobile stations in a mobile vehicle with radio coverage when the mobile vehicle moves within or even outside a cellular radio network comprising a number of fixed base stations. The base station is mobile and it includes first communication means providing radio communication with the mobile stations in the mobile vehicle and second communication means for providing communication with the core network (or the fixed part of the network, e.g. a switching arrangement) and the mobile base station serves a number of mobile cells so that when the mobile cell moves between fixed cells but outside coverage of said fixed cells and when it is within coverage of but not stationary or intending to become stationary, within any fixed cell handovers are not allowed between said mobile and said fixed cells whereas when a mobile cell is (and intending to become) stationary within a fixed cell, handovers are allowed between said fixed and mobile cells respectively.
According to particular embodiment, when a mobile base station is stationary (and possibly also when it intends to become stationary) in a fixed cell, a neighbouring relationship is established and information thereon is provided, particularly in broadcast messages. According to one particular embodiment, when the neighbouring relationship changes, e.g. when a mobile cell ceases to be stationary in a fixed cell or vice versa, the information for example provided in broadcast messages is updated both for mobile cells and for fixed cells. In an alternative embodiment information in the broadcast message is updated only for mobile cells whereas for fixed cells a set of virtual mobile cells is indicated as constant neighbours whereas a switching arrangement controlling mobile cells controls whether handovers are allowed or not.
Therefore also a cellular communication system is provided which includes a number of fixed base stations, each of which serves a number of fixed cells and a number of switching arrangements. The cellular communication system further includes a number of mobile base stations each serving a number of mobile cells provided within mobile vehicles or similar, which comprises first and second communication means for providing communication with mobile stations within said mobile vehicle or similar and the core network (or the fixed part of the network) respectively wherein when a mobile base station is within coverage of a predefined cell, and stationary or intending to become stationary within the fixed cell, the mobile base station is docked with the fixed cell such that handovers are allowed between the fixed cell and the mobile cell.
Particularly information is provided about neighbouring relationships in information messages, for example in broadcast information messages, so that when a mobile cell changes from being stationary within a fixed cell or to being stationary within a fixed cell, information thereon is provided. Particularly updated information is provided both to mobile cells and to fixed cells indicating the current neighbours. Alternatively the current situation is only reflected in information messages, particularly broadcast information messages, provided to the mobile cells whereas a set of virtual mobile cells is constantly indicated to the fixed cell irrespectively of whether the mobile cell is stationary within the fixed cell or outside coverage of the fixed cell etc., a switching arrangement controlling the mobile base station controlling the admissibility of handovers.
In other words, a set of virtual mobile cells are made permanent neighbours to the fixed cell. For the mobile cell the broadcast information message is updated to reflect the current situation. At a handover from a fixed cell to a mobile cell, the call is routed to the switching arrangement, particularly an MSC, handling the mobile cells which uses the information of the present serving cell to match the mobile cell in question. This embodiment is particularly advantageous in that a lower load is incurred upon the fixed cells and the switching arrangement (MSCs) handling them. It is also an advantage since most of the features needed are almost entirely implemented in the mobile cells and the MSC controlling them.
Therefore also a method for providing a mobile station of a cellular communication system comprising a number of fixed base stations each serving a number of fixed cells, with radio coverage when said mobile station is in a mobile vehicle is provided. The method comprises the steps of; arranging a mobile base station in association with said mobile vehicle; providing for communication between the mobile station and the mobile base station using first communication means, e.g. internal antenna means; providing for communication with the core network using second communication means; detecting whether the mobile base station is moving in relation to a fixed cell or whether it is stationary or particularly intending to become stationary within a fixed cell, providing information about neighbouring cells to mobile cells/fixed cells in information messages; allowing handovers between the fixed and the mobile cell when the mobile cell is within coverage of the fixed cell and stationary or intending to become stationary within the fixed cell.
Particularly the method includes the step of updating the information about neighbouring cells when the current situation changes for at least mobile cells. According to one embodiment the method also includes the step of providing updated information about neighbouring cells for fixed cells. Alternatively the method includes the step of permanently indicating a set of virtual mobile cells as neighbours to a fixed cell irrespectively of which is the relation between mobile/fixed cell.